http://code.google.com/p/guava-libraries/ Google’s collections API and many other useful core libraries that help to extend the core Java libraries.
http://www.junit.org/ A good xUnit test framework for Java.
http://www.hibernate.org/ Helps facilitate the storage and retrieval Java domain objects through Object/Relational mappings.
Joda-Time: ………………………….. http://joda-time.sourceforge.net/ Makes dealing with date and time calculations and objects significantly easier.
http://www.springframework.org/ Provides extra functionality for everyday applications but mainly used in the creation and management of web applications.
http://lucene.apache.org/java/docs/ “…is a high-performance, full-featured text search engine library written entirely in Java.”
http://www.jfree.org/jfreechart/ Considered one of the top charting/graph libraries for Java.
http://www.lowagie.com/iText/ PDF creations and manipulation.
http://logging.apache.org/log4j/1.2/index.html A good library devoted to logging.
http://xstream.codehaus.org/ “Out of the box POJO serialization to XML and back again” – Pyrolistical
http://www.eclipse.org/swt/ Libraries for creating extensive UIs in Java.
https://swingx.dev.java.net/ A library for extending the functionality provided by Java’s Swing component.
http://www.mortbay.org/jetty/ “Easy-to-use, full-featured, embeddable web server and webapp container” – David Crow
http://www.jdom.org/ XML and DOM manipulation for Java.
http://seleniumhq.org/ Write tests in JUnit style, then run automated tests against them in Firefox.
http://java.sun.com/javase/technologies/desktop/media/jai/ “… provides a set of object-oriented interfaces that supports a simple, high-level programming model which allows images to be manipulated easily in Java applications and applets. JAI goes beyond the functionality of traditional imaging APIs to provide a high-performance, platform-independent, extensible image processing framework.”
http://velocity.apache.org/index.html Create text (SQL Queries, HTML, etcetera) from predefined templates.
http://jxls.sourceforge.net/ Create XML files from templates.
Google Web Toolkit (GWT):
http://www.dom4j.org/ XML/Xpath/XSLT processing for Java.